In recent years, a polymer electrolyte membrane having a very small thickness, for example, 50 μm or less, has come to be employed as a polymer electrolyte membrane for use in a fuel cell, electrolysis, or electrodialysis, in order to reduce its electrical resistance and thereby further enhance efficiency. However, such a thin polymer electrolyte membrane has a problem that the membrane is not only inconvenient to handle but it is also prone to breakage, because it does not have the physical strength required of a structural supporting member. To overcome this problem, generally it is necessary to reinforce the polymer electrolyte membrane.
In order to increase the mechanical strength of a thin ion-exchange membrane used in a membrane electrode assembly for a polymer electrolyte membrane fuel cell, it is known to provide a seal member which is integrally formed in overlapping fashion around the circumference of a current collector having a five-layer structure comprising an anode current collector, anode catalyst layer, ion-exchange membrane, cathode catalyst layer, and cathode current collector (Japanese Unexamined Patent Publication No. H08-45517). Further, for a polymer electrolyte membrane fuel cell comprising an ion-exchange membrane, gas diffusion electrodes disposed one on each side of the ion-exchange membrane, and current collectors disposed one on the outside of each gas diffusion electrode, it is known to reinforce the periphery of the ion-exchange membrane in the polymer electrolyte membrane fuel cell by forming a resin film around the periphery of the ion-exchange membrane and attaching each gas diffusion electrode in such a manner as to contact or adhere to both the ion-exchange membrane and the resin film (Japanese Unexamined Patent Publication No. H05-174845). On the other hand, in order to prevent breakage of a solid polymer electrolyte membrane in a solid polymer electrolyte membrane fuel cell, it is known to provide a protective film having a frame-like shape formed in a partially overlapping fashion with an electrode and in intimate contact with the peripheral portion of the solid polymer electrolyte membrane disposed at least on one side of the solid polymer electrolyte membrane (Japanese Unexamined Patent Publication No. H05-21077). Further, for a solid polymer fuel cell comprising an ion-exchange membrane and a gas diffusion electrode made up of a catalyst layer and diffusion layer disposed on each side of the ion-exchange membrane, it is known to provide a frame-like reinforcing film having a window opened therein that matches the shape of the catalyst layer disposed at least on one side of the ion-exchange membrane in order to prevent breakage of the thin ion-exchange membrane in the solid polymer fuel cell (Japanese Unexamined Patent Publication No. H10-154521).
Japanese Unexamined Patent Publication No. 2001-129940 describes that when a fluorine-containing copolymer composed of a fluoroolefin, a cyclohexyl group-containing acrylic ester, and a hydroxyl group-containing vinyl ether is applied, together with a crosslinking agent having an isocyanate group, to one and/or both sides of a polyester film and heated, a mold release film having good adhesion to the base film can be obtained. However, Japanese Unexamined Patent Publication No. 2001-129940 provides no description that such a mold release film has excellent adhesion to a polymer electrolyte membrane.